Democratic presidential candidate Barack Obama has written to Tesco chief executive Sir Terry Leahy to urge the UK grocer to start talks with a US trade union over workers' rights at its new California-based business.

In the letter, sent just ahead of the grocer's annual shareholders' meeting on Friday, Obama told the Tesco boss: "It is in your interest to ensure that the communities where you do business and the leaders of the workers in the industry are heard and that their concerns are heard and respected."

Los Angeles-based Fresh & Easy, which Tesco aims to build into a vast chain the size of its UK business, opened its doors last October. It currently has some 60 outlets but has plans for at least 1,000 shops on the west coast. Most of the store workers are part-time.

The UFCW alleges that American workers are not entitled to contracts, which means they could be dismissed without warning, and that staff have to count sick leave as holiday. The report also complains of paltry pension and health benefits.

In Obama's letter, sent to Tesco on Monday, the presidential hopeful contrasted Tesco's stance with the US trade union with the UK, where it recognises unions: "I am aware of Tesco's reputation in Britain as a partner of unions. I would hope that you would bring those values to your work in America."

He added that "ensuring that workers are able to exercise their right to organise and work in safe, rewarding environments" was an important part of his campaign for the Democratic nomination and he intends to continue that campaign in the run-up to the presidential election in November.

He urged Leahy to "reconsider" the US offshoot's policy of not recognising US shopworker unions and tell the Fresh & Easy bosses to meet the union "and other community groups at the earliest opportunity".

It is the second time that Obama has waded into the US trade union's row with Fresh & Easy. Last November both he and Hillary Clinton wrote to Fresh & Easy chief executive Tim Mason urging him to engage with the union.

In the new letter to Leahy, Obama said: "Eight months after my first letter, Fresh and Easy has still not engaged positively with its stakeholders and refuses to meet the UFCW to discuss the principles of partnership. I strongly request you revisit that decision."

UCFW president Joseph Hansen said: "Senator Obama's support for our campaign is welcome and timely. Tesco's UK investors should now be asking Sir Terry Leahy at the company's AGM whether it is really in Tesco's long-term business interests to continue its policy of non-engagement with unions and community groups in America."

He added: "Tesco's business conduct is on the radar of the man who could be America's next president."

A spokesman for Tesco said Leahy would reply to the letter but that Obama's intervention "doesn't change much".

He added: "Our position has been clear for some time. If Fresh & Easy staff want to join the union they can. We are a popular employer, paying good wages and giving good benefits."

He said Obama's assertion that Tesco had not engaged with stakeholders was untrue. "We have engaged with stakeholders. We have engaged with community leaders."

Fresh & Easy, he claimed, was receiving 13 applications for every job advertised and would take on another 750 staff in the next 90 days.